High volume screening assays are commonly employed for detecting the presence of, and quantitatively measuring antigenic materials, antibodies and analytes in biological samples. For example, radioimmunoassay (RIA) techniques are commonly employed for clinical diagnostics. However, RIA procedures are often incompatible with large scale screening programs. Radiotracers, by their very nature, are of limited stability and they require special handling during use, special disposal techniques and sophisticated instrumentation.
Other immunoassay methods currently available include fluorescent and enzymatic techiques. Generally, these assays require a separation step, either by filtration or centrifugation in order to be interpreted. These separation requirements make the assay methods slow and difficult to automate.
Liposomes have previously been reported as useful components for immunoassays. For example, McConnell et al., U.S. Pat. No. 3,887,698, describe the use of liposomes containing stable free radicals in an electron paramagnetic resonance (EPR) monitored immunoassay. Mandle et al., U.S. Pat. No. 4,372,745, describe the use of liposomes as fluorescer containing microcapsules, useful in an immunoassay. This assay requires the use of a detergent such as, Triton X-100 to break the liposomes and release the fluorescent compound. Liposomes have also been employed as a marker carrier in an immunoassay described by Ullman et al., U.S. Pat. No. 4,193,983. Markers used in this assay included fluorescers, enzymes and chemiluminescent compounds.
Kinsky and his colleagues were the first to show that liposomes containing haptenated lipids could bind with an antibody and fix the complement thereof (Haxby et al., Proc. Natl. Acad. Sci. USA, 61 300 (1968); Alving et al., Biochem., 8 1582 (1969); Kinsky et al., Biochem., 8 4149 (1969)). The result was the lysis of the liposomes by the activated complement components.
Cole, U.S. Pat. No. 4,342,826, describes an immunoassay method which utilizes antigen-tagged, enzyme-encapsulated liposomes which are immunospecifically ruptured in the presence of the cognate antibody and an active complement. The assay utilizes the homogeneous phase reaction between the antibody and complement to release the enzyme marker. This complement mediated event has been the focal point for a large amount of literature (for a recent review, see Alving & Richards, Liposomes, Ostro, ed., 209-287 (Marcel Dekker, New York, 1983)).
Recently several noncomplement mediated liposome lytic assays have been developed. For example, binding of the antibody to haptens conjugated to a membrane lytic protein, melittin, blocks and liposome lytic activity of the melittin (Freytag et al., Biophys. J., 45 360(a) (1984)). Binding of the antibody in the Lupus serum to liposomes containing cardiolipin prevents the lysis of the liposome by Mg.sup.+2 ions (Janoff et al., Clin. Chem., 29 1587 (1983)). While no complement is required each of these assays requires either a membrane lytic molecule or ion.
Although the previously described assays may be quite sensitive, they often involve many steps, and are sometimes difficult to reproduce and/or automate. Thus, new and more efficient assays are desirable.